After we crossed the border into Northern Ireland, we drove the few miles of farm road to Derry and parked the car in front of our new home-from-home for the week. Well, we did use it once or twice during the time we were there. One gorgeous day, we took to the road to find the church where my great-grandfather was baptized. Cumber Upper is a wee, bucolic town just ten miles south of Derry.

“Miles?” Marcus asked. “Why are you giving me the distance in miles? You’ve always given me distances in kilometers.”

“For some reason, Google Maps is now giving me distances in miles,” I replied.

That should have been our first clue.

On another day we had to drive to Derry airport to return our second rental car and pick up a third. (For car insurance coverage through our credit card, we can only rent a car for 30 days at a time. Before the 30-day period is up, we have to return one car and rent another; otherwise, we’d have no coverage.) This was another ten-mile drive, but this time along a major thoroughfare.

“Why are these drivers riding my bumper?” Marcus asked. “I’m driving the speed limit.”

I looked in the sideview mirror. There were several cars queued up behind us. One guy passed us and honked, albeit politely. What gives? City drivers! we decided. Always in a hurry to get places.

At the end of the week we checked out of our Derry apartment and started our 60-mile drive to our next destination on the County Antrim coast. We weren’t far into the trip when traffic started piling up behind us again.

“I won’t drive over the speed limit,” Marcus insisted. “I’ve been told by several people that the police here won’t hesitate to pull over a rental car for speeding.”

Why didn’t we have this problem “down south”? I pondered. Why is it just since arriving in Derry? “Hang on a sec. Let me check something.” I pulled out my phone and googled: Are the speed limits in Northern Ireland in mph? Answer (from Wikipedia): Speed limits in Northern Ireland are specified in miles per hour. Those in the Republic use kilometres per hour.

Oh, my! We’ve been driving 60 kph (37.5 mph) in a 60-mph zone! 80 kph (50 mph) in an 80-mph zone! Neither Fodor’s nor the road atlas we picked up, both of which cover both countries, thought to mention this. It’s a wonder people haven’t been making rude gestures as they pass. Obviously, Irish drivers are very patient and kind.

And we thought crossing the border was seamless. Who knew?